Bone marrow-hematopoietic stem cells (HSCs) are functionally defined by their unique capacity to self-renew and to differentiate to produce all mature blood cell types. In general, the process of development from pluripotent progenitors to mature cells with specific functions involves the progressive loss of developmental potential to other lineages. This process has been considered linear in the sense that once a cell has made a developmental choice it cannot revert. The earliest known lymphoid-restricted cell in adult mouse bone marrow is the common lymphocyte progenitor (CLP) and the earliest known myeloid-restricted cell in adult mouse bone marrow is the common myeloid progenitor (CMP), CD34+ cells harbor virtually all in vitro clonogenic potential; however, the CD34+ population is heterogeneous. Only a small fraction (1-10%) of CD34′ cells that do not express mature lineage markers (CD3, CD4, CD8, CD19, CD20, CD56, CD11b, CD14 and CD15; Lin−) have multi lineage (lymphoid and myeloid) developmental potential. The majority of CD34+ cells (90-99%) co-express the CD38 antigen, and this subset contains most of the lineage-restricted progenitors. Deregulation of self-renewal pathways, which are normally tightly regulated in HSCs, has recently been recognized as an important step in leukemic progression.
Myeloid (myelogenous or non-lymphocytic) hemopathies include acute myeloid leukemia (AML) and chronic hemopathies named as myeloproliferative myelodysplasic diseases (MPDs or CMPs). Acute leukemia is characterized by the rapid increase of immature blood cells and can occur in children (ALL) and young adults (AML). Myeloproliferative diseases (MPDs) are a heterogenous group of chronic clonal disorders characterized by cellular proliferation of one or more hematologic cell lines in the peripheral blood, distinct from acute leukemia. Proliferation takes months to years to progress and is distinguished by the excessive build up of relatively mature abnormal blood cells; resulting in increased numbers of granulocytes, red blood cells and/or platelets in the peripheral blood. Myeloproliferative diseases include: Chronic myelogenous leukemia (CML), Polycythemia vera (PV), Essential thrombocythemia (ET), Chronic idiopathic myelofibrosis (Agnogenic myeloid metaplasia (AIM)), Chronic neutrophilic leukemia (CNL), Chronic eosinophilic leukemia/hypereosinophilic syndrome (CEL/HES) and systemic mastocytosis (SM). Myeloproliferative disease may evolve into one of the other myeloproliferative conditions, transform to acute leukemia, or both.
With the exception of chronic myeloid leukemia (CML), the molecular pathogenesis of most chronic myeloproliferative disorders (CMPDs) is not well understood and most CMPD cases have a normal or aneuploid karyotype. However, CML, and some CMPDs are associated with activation of membrane or cytoplasmic Protein Tyrosine Kinases (PTK) by point mutation or chromosomic translocation of respectively the KIT, FLT3 and JAK2 genes or the ABL, PDGFR, 15 FGFRI and FGFR3 genes. Chronic myelogenous leukemia is characterized by t(9;22)(q34;q11) reciprocal translocation (der22 or Ph+ chromosome) and BCR-ABL fusion protein expression. The dysregulated cytoplasmic tyrosine kinase activity of BCR-ABL is responsible for the leukemic phenotype. The BCR-ABL protein is referred to as p185bcr-abl or p210bcr-abl, depending upon the inclusion of the second exon of BCR. p185bcr-abl causes acute leukemia, typically lymphoblastic; p210bcr-abl usually causes CML which may progress to myeloid or lymphoid blast crisis. In polycythemia vera, essential thrombocythemia, and myelofibrosis, the prevalent genetic lesion appears to be a valine to phenylalanine substitution at amino acid position 617 (V617F) within the Janus kinase 2 (JAK2) gene. AML and systemic mastocytosis have been linked with the D816 mutation of the KIT gene. The BCR-PDGFRα or F1P1L1-PDGFRα fusions have been identified in patients with hypereosinophilic syndrome.
Imatinib, a 2-phenylaminopyrimidine molecule, occupies the ATP binding site and inhibits tyrosine phosphorylation of ABL, c-KIT and PDGFRα. Imatinib mesylate (STI571, Gleevec or Glivec) was the first tyrosine kinase inhibitor (TKI) targeted against BCR-ABL to be successfully tested in vivo and is now the gold standard for the treatment of de novo CML in chronic phase (O'Brien et al., N. Engl. J. Med., 2003, 348, 994-1004; Druker et al, N. Engl. J. Med., 2006, 355, 2408-2417). The remarkable efficacy of imatinib has failed, however, to eradicate this disorder, and residual CML disease remains detectable by PCR for most of the patients. Even in patients with complete molecular remission (CMR) for more than two years, molecular relapses within 6 months are observed in half of the patients (Rousselot et al., Blood, 2007 Jan. 1; 109(1):58-60. Epub 2006 Sep. 14).
Imatinib inhibits the tyrosine kinase activity of BCR-ABL and eradicates the proliferating pool of CML cells without being active on CML quiescent cells.
Recent studies have identified a population of rare primitive, quiescent stem cells (LSCs) in all CML patients, whether derived from peripheral blood or blood marrow. These stem cells are predominantly Ph+, express high levels of CD34+ but lack the markers CD38, CD45RA or CD71, and can spontaneously exit Go to enter a continuously proliferating state, to produce Ph+ progeny (Graham et al., Blood, 2002, 99, 319-325; Barnes et al., Cell cycle, 2006, 5, 2862-2866). These cells exhibit an exceptionally high level of inherent insensitivity to conventional chemotherapeutic agents, including imatinib mesylate. Such insensitivity is distinct from acquired resistance, following chronically exposition to the drug whereby resistance to imatinib is frequently mediated by the selection of subclones containing BCR-ABL with point mutations in the ABL-kinase domain. Other mechanisms which have been implicated in clinical resistance (acquired resistance or secondary resistance) to imatinib include over expression of BCR-ABL, amplification of the BCR-ABL oncogene and enhanced drug efflux.
This inherent insensitivity or resistance to drug treatment has important implications for the clinical management of CML−, particularly with regard to relapse following an imatinib-induced remission. However, contrary to acquired resistance to imatinib, the molecular mechanisms responsible for the insensitivity of CML quiescent stem cells are not known. Therefore, there are no obvious molecular targets and no rational choices can be made regarding which agents to combine with imatinib to target the CML quiescent stem cells. Several approaches have been used to try to improve the effectiveness of imatinib; intermittent exposure to granulocytecolony stimulating factor (G-CSF) and vaccination with T peptides able to induce specific cytotoxic T-cell response. Accordingly, improved therapies for preventing hematological cancer relapse are needed.